1. Field of the Invention
This invention relates to a system in which analog signals are transformed into digital signals by encoding apparatus that includes an amplitude adjustment means actuated for a limited time to permit analog signals of greater amplitude to be digitally encoded. The invention further relates to transformation of the digital signals back into analog form, including means actuated for a limited time to allow transformation of properly encoded signals back to greater amplitude analog signals. In either type of transformation certain digital signals are used to control the amplitude adjustment apparatus to expand the amplitude of analog signals that may be handled without correspondingly expanding the number of digits that must be used.
2. The Prior Art
When it is desired to process analog signals by means of apparatus capable of handling only digital signals, the analog signals must be converted into digital signals. Basically, this involves selecting the number of digits that one is willing to use, and then assigning a number of levels to the analog range, each level corresponding to a digital increment. For example, if four digits are to be used, the maximum number of analog steps can be fifteen, since the decimal equivalent of the binary number 1111 is 2.sup.4 -1, or 15. This maximum decimal value is then considered to be equal to the maximum analog signal amplitude and any signal value below that maximum corresponds to a certain digital number. For example, the first increment corresponds to the digital number 1, the second to the digital number 10, and so on.
The analog signal can be reconstructed by transforming the digital signal back into analog form. The reconstructed analog signal may not have a smooth variation from one level to the next unless it is possible to use a smoothing filter. However, the greater the number of increments, the more closely the waveform of the reconstructed analog signal will correspond to the original signal, provided there is time to carry out the quantizing procedure at each step.
The actual magnitude of the analog signal does not affect the quality of the reconstructed signal; if the encoding apparatus is designed to encode an analog signal having a maximum peak value of one volt (an arbitarily chosen value) and the actual signal is 10 volts, it has been the practice heretofore to attenuate the actual signal to the acceptable value. On the other hand, if the actual signal is only a fraction of a volt, the practice has been to amplify it to the desired value. In any case once the amplitude of the analog signal was adjusted so that the maximum value of the signal could be accommodated by encoding apparatus with peak values of the analog signal using all or almost all of the available digital increments, the number of available increments was determined by the number of binary digits, and it was impossible to accommodate peaks of the analog signal that exceeded the predetermined level selected as the maximum. It would be possible to increase the number of binary digits but before doing so it is necessary to take into account the frequency range of the apparatus, such as a transmission line, through which the binary signals would have to be passed. A larger number of digits means more complex and more expensive equipment or a limitation in the frequency range of the analog signals.
In co-pending application entitled TRANSMITTING APPARATUS USING A/D CONVERTER AND ANALOG SIGNAL COMPRESSION AND EXPANSION, filed June 21, 1974, Ser. No. 481,804, by Jun Takayama and Takenori Sonoda, a system is described for controlling the amplitude of the analog signal to reduce its peak values before encoding it into a digital form and to expand it when it is reconstructed into analog form. The amplitude is changed by detecting a first digital signal corresponding to the maximum permissible digital value for the encoder and using that detected signal to compress the amplitude of the analog signal to be encoded. The same digital value can then be used in decoding the signal to expand the reconstructed analog signal.
A second digital value corresponding to approximately half the maximum permissible, or full scale, analog value would then be used to return the amplitude of the analog signal, before encoding or after decoding, to the original value.
However, transmission of the encoded digital signals may result occasionally in dropping out the important amplitude adjustment signals, either the compression or the expansion signal, with an attendant distortion of the reconstructed analog signal.
It is one object of the present invention to provide conversion apparatus for use in analog-to-digital and digital-to-analog conversion systems to accommodate higher peak values of the analog signal without increasing the number of binary digits by using a pattern, or code of the binary signal to change the amplitude of the analog signal in conversion, and in making these conversions, to take advantage of the fact that the signal does not remain long at its peak value, thus making it possible to control the time that the amplitude is expanded.
Further objects will become apparent from the following specification together with the drawings.